1. Field of the Invention
The present invention relates to a magnetic bearing for rotatably supporting a rotor without involvement of any physical contact therewith, by utilization of magnetic attraction force.
2. Description of the Related Art
An example of conventional magnetic bearing is described in, for example, Japanese Patent Examined Publication (Kokoku) No. Hei-5-17406. In this example, a radial magnetic bearing comprises a radial stator and a rotor. The radial stator is configured so as to be able to comprise three pairs of magnetic poles, and each pair has windings coiled so as to form a north pole and a south pole in the form of a horseshoe within a radial plane. The rotor is attached to a rotor shaft and is formed by stacking annular steel plates into a multilayer element. In this example, a large bearing loss, which will be described later, arises in the radial magnetic bearing. The electromagnetic polarity of the radial magnetic bearing alternates with respect to the direction of rotation, so that great hysteresis and eddy-current losses develop in the multilayered annular steel plates attached to the rotor shaft, thus generating heat in the rotor shaft. Such heat generation results in a loss of control power supplied to the magnetic bearing.
A radial magnetic bearing which prevents such a power loss is described in U.S. Pat. No. 4,983,870, wherein the electromagnetic polarity of the radial magnetic bearing does not alternate with respect to the direction of rotation. An eddy current loss is usually caused by a change in the intensity of the magnetic field stemming from repetition of regions where magnetic polarity exists and regions where magnetic polarity does not exist, within a peripheral surface of the rotor shaft with respect to the circumferential direction. In the foregoing radial magnetic bearing, the multilayered electromagnetic steel plates--through which an eddy current passes--are isolated from one another with respect to the circumferential direction. Hence, substantially no eddy current develops in the multilayered electromagnetic steel plates, thus considerably reducing a bearing loss.
However, a large current loss still exists even in the above radial magnetic bearing. This type of radial magnetic bearing involves constant flow of a bias current which provides a bias magnetic flux in order to effectively prevent an eddy current loss, thus resulting in an additional power loss.
A conceivable measure for eliminating a necessity for the bias current to flow through the magnetic bearing is a magnetic bearing which uses a permanent magnet for generating a bias magnetic flux. One example of such a magnetic bearing is described in Japanese Utility-Model Unexamined Publication No. Hei-2-87120; namely, a thrust magnetic bearing which employs a permanent magnet so as to form a portion of an electromagnetic pole, as means for eliminating a necessity for inducing a bias current for generating a bias magnetic flux. Another conceivable measures for embodying a radial magnetic bearing employing a magnet for diminishing a bearing loss is to apply the thrust magnetic bearing, as described in Japanese Utility-Model Unexamined Publication No. Hei-2-87120, which supplies a bias magnetic flux through use of a permanent magnet, to the radial magnetic bearing as described in U.S. Pat. No. 4,983,870.
In a commonly-known control circuit for controlling the above-described type of magnetic bearing, which uses a permanent magnet, a compensation circuit is activated such that an object of control is to be positioned in accordance with a position instruction value, and a control current supplied from a power amplifier to a magnetic coil is adjusted to an appropriate value. In this type of control circuit, the control current flowing to the magnetic coil is integrated, and the integration result is fed back to the compensation circuit, thus shifting the object of control to a position where a balance exists between the gravitational force acting on the object and the attraction force exerted by the magnet. In the end, the electric current flowing through the magnetic coil is made virtually zero.
In the magnetic bearing, which employs a magnet and the above-described control circuit, the rigidity of the bearing is zero. If variable load, such as cutting load corresponding to continuous imparting of static load, acts on the shaft, the position of the shaft gradually varies. In contrast, if dynamic load acts on the shaft, the position of the shaft is changed, thus resulting in the probability of an accident such as a collision between the rotor shaft and the stator.
FIG. 3 is a block diagram showing a control circuit for use with a magnetic bearing, which uses a magnet, described in Japanese Utility-Model Unexamined Publication No. Hei-5-10822, which serves as an example of the control circuit for use with the conventional magnetic bearing using a magnet. In this control circuit, a compensation circuit 301 outputs an instruction signal to a power amplifier circuit 302, which in turn actually drives an object of control 303. At this time, a VZP feedback circuit 304 is activated, and a load signal is integrated by an integrator 305, to thereby control a load current signal so as to make the load current signal zero. An amplitude limit circuit 306 and a gain setting circuit 307 control a feedback to be output from the VZP feedback circuit 304 so as not to become excessive, thus preventing an accident, such as a collision between the rotor shaft and the stator, which would otherwise be caused when a static variable load greater than a predetermined level is imparted to the shaft. The feedback signal from the VZP feedback circuit 304 and a displacement signal 308 are negative-feedbacked to the compensation circuit 301.
If variable load exceeding the range of VZP operation is imparted to the shaft, the position of the shaft becomes undefined within a predetermined range, as a result of which the control circuit encounters difficulty in retaining the shaft at a precise target position.